The present invention relates to a method and an apparatus for cleaning an ink jet printhead having a nozzle face in which at least one printhead nozzle is formed.
A typical ink jet printhead comprises a plane nozzle face in which a linear array of individual nozzles are formed. The printhead further comprises means for supplying liquid ink to the nozzles and for generating pressure pulses in the ink liquid, so that ink droplets are expelled from the nozzles. Since the nozzles tend to become clogged with dried ink or foreign matter, it is necessary to clean the nozzles from time to time. Further, the nozzle face surrounding the nozzle orifices opening may become soiled with dust or the like, and this may deteriorate the process of droplet formation and/or influence the direction in which the ink droplets are jetted out. For this reason it is also necessary to clean the nozzle face at least in the vicinity of the nozzles.
U.S. Pat. No. 5,574,485 discloses a cleaning head which is disposed in front of the nozzle face and can be moved along the linear array of nozzles in a cleaning operation. This cleaning head has a suction nozzle facing the printhead with a small gap formed between the end of the suction nozzle and the nozzle face of the printhead. The suction nozzle is connected to a suction device such as a vacuum pump and can be aligned with the individual printhead nozzles. A cleaning liquid is fed to the cleaning head and pumped against the nozzle plate. This cleaning liquid which dissolves the ink is sucked into the suction nozzle in order to scavenge and clean the printhead nozzle.
The cleaning head further has an ultrasonic liquid wiper juxtaposed to the suction nozzle so that it also confronts the nozzle face of the printhead. The wiper is formed by a tubular ultrasonic transducer the front end of which also forms a small gap with the nozzle face. The tubular transducer defines a supply channel through which a cleaning liquid, e.g. a solvent, can be supplied into the gap. The cleaning liquid forms a liquid bridge between the end of the transducer and the nozzle face. This liquid bridge is stabilized in the gap by the surface tension of the liquid and moves together with the transducer when the cleaning head is scanned along the nozzle array, so that the nozzle face is wiped with cleaning liquid. In order to enhance the cleaning effect, the transducer is energized so that ultrasonic waves are created in the liquid bridge.
U.S. Pat. No. 5,412,411 discloses an ink jet printer in which the whole nozzle face of the printhead can be immersed in liquid ink contained in a tank, whereby the ink in the tank is used for capping and cleaning the nozzle face.
It is an object of the present invention to provide a method and apparatus with which an ink jet printhead can be cleaned easily and efficiently.
According to the present invention, a suction nozzle is disposed in front of the nozzle face, so that it forms a small gap therewith, and ink is sucked out of the nozzle printhead and is caused to spread or flow in said gap in a direction parallel to the nozzle face, wherein pressure waves are generated in the ink volume which forms a liquid bridge (54, 56) between the nozzle face (28) and the suction nozzle (38) by activating the printhead (10) whereby ink is ejected from the printhead nozzles (16) that open into the gap (44).
Thus, according to the present invention, the ink itself is used not only for scavenging the nozzles but also for cleaning the nozzle face. Since the ink serving as cleaning liquid is supplied directly via the printhead nozzles, it is not necessary to provide separate supply means for the cleaning liquid. In addition, the cleaning or scavenging of the nozzles and the cleaning of the nozzle face surrounding the nozzle orifices can be achieved very efficiently in a single operation. The spreading of the ink in the gap is highly promoted by activating only the printhead nozzles that are currently facing the suction nozzle, so that ink droplets are actively ejected into the gap and the ink can accumulate in the gap.
The invention is useful for cleaning all sorts of inkjet systems, whether water based, solvent based or hotmelt ink based. In the latter case cleaning is performed when the ink is in a molten state.
Several effects can be used either alone or in combination for causing the ink supplied via the printhead nozzles to spread in the gap. For example, such spreading can be caused by capillary action which occurs when the gap is made small enough and the wetting angle of the ink with the nozzle face of the printhead on the one hand and the end face of the suction nozzle on the other hand is sufficiently small.
Another way to cause a spreading of the ink is to make the end face surrounding the mouth of the suction nozzle sufficiently large, so that it covers a plurality of printhead nozzles. Then, when air is drawn into the suction nozzle, a low pressure zone is created in the gap between the suction nozzle and the nozzle face of the printhead, so that there is an extra force sucking the ink from a plurality of printhead nozzles at a time which causes the ink to flow through the gap towards the mouth of the suction nozzle.
The suction nozzle may be formed as or combined with a vibrator or ultrasonic transducer for generating pressure fluctuations in the ink contained in the gap, thereby enhancing the cleaning effect.
When the suction nozzle is scanned along the array of printhead nozzles, the ink volume contained in the gap will move together with the suction nozzle so that a stripe shaped portion of the nozzle face containing the nozzles is wiped with ink. When the suction nozzle is moved to an inoperative position, e.g. at one end of the nozzle array where no printhead nozzles are present, and the suction device is still kept operative for some time, the ink will be removed from the gap without remnants.
The method and apparatus described herein are suitable for solvent-based inks, but are also particularly useful for hot-melt ink which is solid at room temperature and is kept at a temperature above its melting point, e.g. at about 100xc2x0 C., when the printer and/or the cleaning device is operating. The method can also be used to remove air from the printhead.